1. Field of the Invention
The present invention relates to an elastomeric article which is designed to be used primarily in a flexographic printing system. Flexographic printing is a method of rotary letterpress printing which employs flexible elastomer printing plates. In this system the ink is transferred directly from the raised surfaces of the elastomeric plate to the printing stock.
By way of contrast, many references in the prior art disclose a resinous relief plate which is designed to be used in a letterset printing system. Letterset printing is a method of rotary printing which employs an elastomeric printing blanket. In this system the ink is transferred from the raised surfaces of the resinous plate to a rubber blanket which in turn transfers the inked image to the printing stock.
The main difference between the flexographic and letterset printing methods is that the transfer of the inked image is direct in the former and indirect in the latter.
In both the flexographic and letterset systems noted above, it is necessary to have a resilient member at the point of ink transfer to the printing stock. In flexographic printing the elastomeric plate is the resilient distortable member whereas in letterset printing the rubber blanket is the resilient distortable member. Thus a resinous printing plate (as described in the prior art) is not generally used in a flexographic system because it does not possess the suitable resilient properties necessary to effect a smooth uniform transfer of ink directly from the printing plate to the material to be printed.
The utility of a printing plate is the sum total of its properties, that is, in order to be commercially useful, a plate must possess the proper strength, durability, resistance to tear, resistance to solvents, hardness and swell characteristics. Many of these properties mentioned are a function of the polymer matrix which is used as the predominant constituent in the compositions which comprise the plate. After careful evaluation, the elastomers set forth hereinafter were selected as suitable for the matrix polymer for the instant invention since plates made from them possessed the desirable properties noted above, especially with respect to resistance to printing ink solvents.
The distinction between resinous and elastomeric polymers as used in the printing arts is not one of mere form but rather is one of significant substance.
"Properties and Structures of Polymers" by A. V. Tobolsky, pp. 771-78, John Wiley & Sons, Inc., Publishers (1960) discloses criteria for determining whether a polymeric material is a resin or an elastomer. Those polymeric materials which at ambient temperatures possess glassy characteristics and a Young's modulus in excess of 10.sup.9 dynes/cm.sup.2 are designated resins. Conversely, those polymeric materials which at ambient temperatures are leathery or rubbery in nature and which possess a Young's modulus between 10.sup.5 and 10.sup.9 dynes/cm.sup.2 are designated elastomers. This distinction is presented to emphasize that there is no basis for considering a resin and an elastomer as "equivalents" for the purpose of the present invention.
2. Description of the Prior Art.
There are no known post-treatments for flexographic plates of the type used in the present invention which both detackify and protect the printing surface from the phenomenon referred to and described hereinafter as "post-hardening due to light." Treatment with aqueous chlorine (acidified laundry bleach) has been used for detackification; however this treatment offers no protection from post-hardening due to light.